Research in the immunology and immunopathology of lymphatic filariasis has lagged behind other areas, partly because of the absence of a syngeneic, immunologically-defined rodent host. Although many filariasis study have passively followed the kinetics of various parameters of B and T cell activity during infection, the mechanism of in vivo destruction of lymphatic parasites has never been demonstrated. The Lewis rat, developed as a model for protective responses against microfilariae, larvae or adult worms, promises to be valuable in such studies and has several key advantages over previous models. The objective of the proposed research is to define the Lewis rat infected with Brugia pahangi as a model for such investigations, with the major emphasis on demonstrating protective responses against the infection. Patterns of microfilaremia will be correlated with tissue destruction of microfilariae and reinoculation studies will attempt to provoke strong resistance against larval or adult worms and to define the optimum conditions for these responses. Baseline data on parasite development, life cycle and the histologic response of the host will be gathered and compared with that from established models such as the cat and the bird. Relevant aspects of the life cycle include larval growth rate and onset of moult, larval mortality, location, size, sex ratio and rate of fertility among adult worms, prepatent period and patterns of microfilaremia. Significant pathological changes, particularly those of interest in human disease will be identified.